The structures of NADP ion-specific isocitrate dehydrogenase (theo-Ds-isocitrate: NADP ion oxidoreductase(decarboxylating), EC 1.1.1.42) from the bacterium Azotobacter vinelandii and from porcine heart will be studied by X-ray diffraction methods. The goals of this project will be to understand the mechanism of action of this enzyme at the atomic level and to investigate the structure-function relationships of NAD ion- and NADP ion -binding dehydrogenases. The evolutionary relationships of isocitrate dehydrogenase with the NAD ion -specific dehydrogenases whose tertiary structures are well known will also be studied. NADP ion -specific isocitrate dehydrogenases are single chain polypeptides of molecular weights 37,500 to 92,000 daltons, depending on the source. The two enzymes to be initially studied have molecular weights of 80,000 daltons for the A. vinelandii enzyme and 58,000 daltons for the porcine heart enzyme. Crystals of bacterial enzyme have been obtained. They are tetragonal with two molecules per asymmetric unit. Native data to 5.4 A resolution have been collected. The rotation function indicates three possible orientations of the non-crystallographic two-fold axis relating the two molecules. The isomorphous replacement method of protein crystallography coupled with the molecular replacement method will be used to solve these structures.